btrfs: use smaller type for btrfs_path reada

The possible values for reada are all positive and bounded, we can later
save some bytes by storing it in u8.
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ctree.h

@@ -596,7 +596,7 @@ struct btrfs_path {
 	int slots[BTRFS_MAX_LEVEL];
 	/* if there is real range locking, this locks field will change */
 	int locks[BTRFS_MAX_LEVEL];
-	int reada;
+	u8 reada;
 	/* keep some upper locks as we walk down */
 	int lowest_level;